The present invention relates to methods and devices for treatment of wastewater. More particularly, the methods of the present invention are designed for the biological removal from wastewater of contamination in the from of insoluble suspended solids and soluble and insoluble organic and inorganic material including nitrogen and phosphorus nutrients.
Wastewater which is produced every day by domestic activities (wastewater from bath, kitchen, shower, toilet, washing-machine, etc.) and in industry, must be purified for both organic materials and inorganic materials such as nitrogen compounds and phosphorus nutrients, before being discharged in surface waters or being treated for reuse (irrigation, cleaning or process water).
Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or xe2x80x9copenxe2x80x9d systems) rather than as xe2x80x9cbatchxe2x80x9d or a series of periodic treatment processes in which treatment is carried out on parcels or xe2x80x9cbatchesxe2x80x9d of wastewaters. While most wastewater treatment processes are continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are routinely handled as periodic batch operations.
Wastewater treatment, however, can also be categorised by the nature of the treatment process operation being used; for example, physical, chemical or biological. A complete treatment system may consist of the application of a number of physical, chemical and biological processes to the wastewater. The present invention relates to biological wastewater treatment. Biological treatment methods use micro-organisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products. More micro-organisms, or sludges, are formed and a portion of the waste is converted to carbon dioxide, water and other end products. Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on presence of dissolved oxygen.
Basics of biological wastewater treatment are given hereinafter.
Aerobic biological wastewater treatment may be carried out in an aerobic activated sludge reactor, where activated sludge (flocculated aggregates of micro-organisms) is aerated and fed with wastewater. The activated sludge is a microbial mass which has increased biological activity through aeration in a vessel.
The suspended and dissolved organic matter of the wastewater is in a first enzymatically induced step, adsorbed and absorbed by the micro-organisms in the sludge flocks (the accumulation process).
The accumulated or stored substrates are then oxidised in carbon dioxide and water, while producing energy (the dissimilation process).
The generated energy is used to take up the substrates and generate new micro-organisms (the assimilation and regeneration processes).
In a clarifier, the mixture of the sludge and treated wastewater is separated by gravitational sedimentation in a bottom sludge blanket and supernatant effluent. Depending on the technology, the bottom sludge is recycled to the aeration reactor or re-activated later in the same compartment.
Next to organic matter, wastewater usually also contains nitrogen and phosphorus compounds. These compounds exist under different forms, like particulate or dissolved organic bound nitrogen and phosphorus, and dissolved ammonia, nitrates, nitrites and ortho-phosphates.
European and world-wide legislative bodies have issued strict discharge limits for nitrogen ( less than 10 mg N/I) and phosphorus ( less than 1-2 mg P/I).
For this reason enhanced biological nitrogen and phosphorus removal is often incorporated in biological wastewater treatment processes.
To activate biological nutrient removal in wastewater treatment plants, specific conditions should be applied to the activated sludge micro-organisms. A distinction can be made between assimilative and dissimilative nutrient removal.
Hydrolysis of organic bound nitrogen or phosphorus to dissolved ammonia and ortho-phosphates is a process that is important to make the nitrogen and phosphorus available to the micro-organisms.
During the assimilation process the micro-organisms incorporate nitrogen and phosphorus in new cell material according to a specific ratio based on an ideal cell material composition. This ratio is estimated to be approximately 100:5:1 as BOD:N:P. This is a rather conservative value and depending on the F/M ratio, a measure of food provided to micro-organisms in an aeration tank, and thus depending on sludge loading. BOD is the Biological Oxygen Demand, the quantity of oxygen necessary for breakdown of readily decomposable organic matter added to water (or a liquid mix such as liquid manure).
If the nutrient content of the wastewater is low, then nutrients should be added to obtain a good working activated sludge. Addition of nutrients is often necessary when treating high organically loaded wastewaters from the food industry.
In general the assimilative nutrient removal is depending on the sludge production and thus on the organic loading of the plant. Since sludge production is normally kept as low as possible, the assimilative removal capacity is rather limited.
The specific conditions for dissimilative enhanced biological nitrogen and phosphorus removal processes are different. Therefore these processes are described separately.
The dissimilative nitrogen removal is a process that is carried out by a specific group of bacteria in the activated sludge during different environmental conditions or states.
A first step in the dissimilative nitrogen removal is called nitrification or ammonia oxidation, carried out by autotrophic nitrifying organisms during aerobic conditions. The nitrifying micro-organisms in the activated sludge oxidise ammonia to nitrite and nitrate. This process uses oxygen, according to the following reaction (nitrification reaction):
ammonium-N+oxygenxe2x86x92nitrate-N+water+protons+energy 
A second step in the dissimilative nitrogen removal is called denitrification, carried out by facultative anaerobic heterotrophic bacteria. During denitrification the nitrate is transformed via nitrite and several intermediates to atmospheric nitrogen. The denitrification reactions are as follows:
nitrate-N+electrons+protonsxe2x86x92nitrogen(gas)+water+energy 
carbon+waterxe2x86x92carbon dioxide+electrons+protons 
In this process, nitrates are used for the oxidation of carbon, and this process provides the energy for cell growth.
Denitrification is achieved under low or zero dissolved oxygen concentrations (anoxic phase). As carbon source for denitrification, raw wastewater should be used to the maximum.
Oxygen and alkalinity are partly recovered during this process.
Important control parameters for dissimilative nitrogen removal are the aerobic and anoxic retention time of the bacteria, the availability of rapidly biodegradable organic substrates (volatile fatty acids), pH and temperature and especially the overall sludge retention time in the system (typically  greater than 10 days at 15xc2x0 C.).
Biological phosphorus (BioP) removal is achieved in a sequence of anaerobiosis (the presence of life in the absence of air or free oxygen) and aerobiosis (the presence of life in presence of air or free oxygen).
Strictly aerobic organisms can survive in anaerobic conditions on short chain fatty acids. These acids are taken up using energy from hydrolysis of polyP (polyphosphate). Finally polyP is hydrolysed, ortho-phosphates are released in the liquid and an internal C-source (carbon source) is formed.
Oxidation of the internal C-source in the subsequent aerobic sequence directly provides energy for p-uptake, polyP-storage and cell-growth. To obtain a net P-removal the uptake should be higher than the initial release.
Important control parameters for BioP removal are the aerobic and anaerobic retention time of the bacteria, the availability of rapidly biodegradable organic substrates (volatile fatty acids), the nitrate concentration (as low as possible), pH and temperature and especially the overall sludge retention time in the system (typically  greater than 5 days).
Anaerobic digestion is a biological treatment process, where organic compounds are anaerobically transferred into methane gas. It is depending on complex interactions between several involved groups of micro-organisms, which interactions take place in anaerobic sludge granules that are normally present or selected in an up flow anaerobic sludge blanket system.
First the complex particles and dissolved polymers are hydrolysed to simpler soluble molecules. These products are then used by fermentative micro-organisms, to produce mainly volatile fatty acids (VFA), aldehydes, alcohols, carbon dioxide and hydrogen. In this acidogenesis phase (phenomenon by which organic matter is transformed into organic acids), partial removal of suspended solids can be carried out by pre-sedimentation.
In the subsequent acetogenesis phase (the second stage of decomposition of organic materials, whereby molecules are acted upon by anaerobic bacteria to produce volatile organic acids, carbon dioxide and hydrogen), most of the fermentation products must be further degraded by the acetogens (also known as acetic acid bacteria) to yield acetate, H2 and may be CO2.
The acetogens grow close to methanogenic bacteria (organisms that generate methane as a metabolic end product), because they can only grow at very low hydrogen concentrations.
The final step of the anaerobic digestion is carried out by the methanogenic bacteria and is the formation of methane gas from acetate and from hydrogen and carbon oxide. The gas formation is important for mixing purposes of the sludge blanket and the wastewater.
Important control parameters for anaerobic digestion are temperature (mesophilic bacteria, 30xc2x0 C.), pH (alkalinity recovery by effluent recycle) and sludge retention time. Sludge retention is high because of the low growth rate of the methanogenic bacteria population. The acidogenic bacteria, however, have a much higher growth rate, which is the reason that a two-stage process may be applied. Also pre-sedimentation during hydrolysis is especially important to avoid too low sludge retention times.
Conventional activated sludge wastewater treatment technology in its most generic form comprises an activated sludge reactor connected with a separate sedimentation tank. There are thus different dedicated tanks for reaction and sedimentation, which are separated in place.
In the activated sludge reactor the Mixed Liquor Suspended Solids (MLSS), the total solids (organic and inorganic) in the reactor, is aerated and fed with wastewater. Many different aeration systems can be used, depending on the type of wastewater, the size of the wastewater treatment system and the necessary oxygen transfer efficiency.
The sludge-water mixture flows under gravity to the sedimentation tank, where settlement of the sludge flocks separates the micro-organisms from the treated water. The clarified supernatant layer or effluent is discharged via a weir system. A sludge blanket at the bottom part of the sedimentation tank, a distance under the surface of the final clarifier where settle sludge is lying, is slowly pushed to a suction pit of sludge recycle and sludge waste pumps by a slowly turning raking system. The sludge recycle pump transports the sludge back to the activated sludge reactor. The excess sludge is wasted to a sludge treatment unit to maintain a constant sludge concentration in the system.
In cases where the nutrients nitrogen and phosphorus have to be removed from the wastewater besides the organic content, extra reactors to introduce anoxic and anaerobic circumstances are needed. The introduction of these extra reactors implies also recirculation of sludge from the anoxic to the anaerobic reactor or from the aerobic to the anoxic reactor, depending on the chosen system.
Other alternatives for the introduction of nutrient removal are known as the oxidation ditch concept. The oxidation ditch is a modified form of the activated sludge process. The ditch consists of two channels placed side by side and connected at the ends to produce one continuous loop of wastewater flow, with a brush rotator assembly placed across the channel to provide aeration and circulation. In this system a continuous cycling of sludge in a closed loop takes place. There is no separation by walls in this reactor. Nutrient removal can be obtained by introducing anoxic and anaerobic zones in the loop by controlling the oxygen transfer level and influent distribution. In this way no sludge recycling devices are necessary to obtain the anoxic and anaerobic zones.
As in the compartmentalised conventional systems, sludge-effluent separation takes place in a separate sedimentation tank.
In general the design of activated sludge wastewater systems is very much depending on the type of wastewater to be treated and the client (industrial or municipal wastewater, industry or government as a client), the available space (city or country side), the area of implantation (nearby sensitive ecosystems, nearby sea, river), country economics and also origin of the design and engineering companies.
Main advantages of this conventional activated sludge wastewater system are the continuous influent and effluent flow rate and the constant water level. Main disadvantages are the separation in space of the active and sedimentation compartments, the necessary devices for sludge raking and recirculation, the mostly circular configuration of the sedimentation tanks and the rather high footprint (or surface) for this type of plants.
Batch reactor activated sludge wastewater treatment can be carried out in a single reactor or in multiple reactor set-ups. Each reactor compartment has hereby two main functions, (1) a biological treatment function (oxidation, nitrification, denitrification, and phosphorus removal) and (2) a sedimentation function (solids-liquid separation).
The absence of a separate sedimentation tank means that no recirculation pumps or screws and piping are necessary. Also no bottom-raking device is necessary.
A single reactor is operated according to a fill and draw principle (FandD), which is opposed to a reactor through which liquid flows continuously at its normal rate of flow as in the conventional activated sludge system. In a batch reactor, several phases are followed in a cyclic pattern according to a specific time interval. In general following phases can be distinguished: Fill, React, Settle, Decant (or Draw) and Idle. Excess solids can be discharged during the Settle phase. Because these subsequent phases are continuously repeated, these systems are often named as intermittent or cyclic operating systems. Reaction and sedimentation are separated in time.
When a single reactor compartment is used (FandD), influent feeding and effluent discharge are discontinuous and therefore a holding tank is necessary.
With two or more reactor compartments the wastewater treatment system may be catalogued to the sequencing batch reactor technology (SBRxe2x80x94biological treatment and settlement of solids are combined in one reactor). Each tank is then operated according to the same cycle of subsequent phases (Fill, etc.), but in such a non-synchronic, out of phase way that continuous influent feeding and effluent discharge are possible over the overall system.
During operation the filled reactor volume varies between top water level and bottom water level. For this reason the sequencing batch reactor systems or cyclic operating systems are collectively regarded as variable volume systems.
Main advantages of the cyclic operating FandD- or SBR-systems are the easy and compact construction without sludge raking and recirculation devices, the control in time possibilities that allows exact control of all specific phases and change in substrate gradients, resulting in microbial selection of well settling sludge. Main disadvantages of cyclic operating FandD- or SBR-systems are especially the variable volumes (level changes) and the intermittent influent feeding and effluent discharge. Overall influent and effluent flow rate may be constant when several (at least two) tanks are operated according to the same cycle of subsequent phases in a non-synchronic, out of phase way. However for each tank still intermittent feeding and discharge occurs with all its disadvantages, such as for example higher pipe diameters, pump capacities and aeration system capacities.
Therefore, both conventional activated sludge systems and cyclic operating activated sludge systems (FandD, SBR) have their advantages and disadvantages. Several attempts have been made to combine the advantages of both system types in one hybrid continuous cyclic operating system.
Development of these hybrid systems, which can be generally classified as continuous cyclic operating activated sludge systems, evolved in several different directions.
The hybrid continuous cyclic operating activated sludge system can in general be described as a system with separation of reaction and sedimentation in time, while also having a continuous inflow and outflow. The known hybrid continuous cyclic operating systems can be divided into two subclasses of hybrid systems:
semi-hybrid cyclic operation systems
hybrids with asymmetric functional cycle
The semi-hybrid cyclic operation systems have more functional characteristics of the continuous conventional system than of the cyclic operating systems.
In U.S. Pat. No. 3,977,965 a system is described whereby reactions are separated in time, and whereby a dedicated sedimentation tank is provided. Two hydraulically connected compartments which are alternately fed, aerated and mixed, are described. Sludge from one of these two compartments always flows to the one dedicated sedimentation tank. Thickened sludge is recycled from the bottom of the sedimentation tank to one of the two active compartments.
In U.S. Pat. No. 5,902,484 a system is described where the active compartment(s) are dedicated compartments (for reaction only) and where only the sedimentation tank or tanks are alternated in time between sedimentation and aeration (resuspension and activation of the thickened sludge to avoid the use of sludge raking systems). The thickened sludge is recycled to the active reactor(s) for redistribution of the sludge.
Known hybrid systems with asymmetric functional cycle have reaction and sedimentation separated in time, continuous inflow and outflow, and different functional units and equipment (e.g. 2 outer compartments and 1 different middle compartment). Such a system is for example described in U.S. Pat. No. 4,179,366, where, at one moment in time, a first outer tank and the middle tank act as activation tanks while the second tank acts as a post-clarification tank. At another moment in time, the flow through the tank is reversed, so that the second outer tank now acts as an activation tank, and the first outer tank as a post-clarification tank. The middle tank continues to operate as activation tank.
A hybrid cyclic operating system with almost the same asymmetric functional cycle is described in U.S. Pat. No. 3,977,965. The only difference is the type of tanks used (ditches are used in stead of rectangular tanks) and the integration of nitrogen removal (generic principle) by adding a fed-stirred active phase next to the fed aerated phase).
Also in DE-3147920 and in BE-899757, similar hybrid systems are described.
All systems in above patents typically consist of an embodiment (rectangular tanks, square tanks or ditches) divided by two walls (with opening) in three compartments. The outer compartments serve alternately as active (fed, aerated, stirred) and sedimentation compartments, the middle compartment always acts as an aeration compartment. In one phase, wastewater-sludge mixture flows from one aerated, fed and/or stirred outer compartment to the middle aerated compartment and finally to the opposite outer sedimentation compartment (where treated effluent leaves the system). In the subsequent main phase, wastewater-sludge mixture flows in the opposite direction from the newly active outer (former sedimentation) compartment back to the middle compartment and to the new outer sedimentation (former fed and aerated) compartment, where treated effluent leaves the system. Waste sludge can be evacuated out of the actual sedimentation compartment.
All these systems describe an intermediate phase where the one outer compartments is prepared to shift function from active compartment to sedimentation compartment.
The functional cycles describe sludge transfer from one outer compartment via the middle compartment to the opposite outer compartment and back in the opposite direction. The functional cycles give rise to an unequal sludge concentration distribution over the three compartments, which is responsible for an unequal loading over the three compartments. The outer compartments are higher loaded and the middle compartment is lower loaded, which generates a not so effective efficiency of the aeration system. The sludge concentration in the outer compartments is higher than in the middle compartment, hindering the sedimentation process.
It is an object of the present invention to provide a method for biological wastewater treatment which does not present the disadvantages mentioned before.
In particular it is an object of the present invention to provide a method for biological wastewater treatment whereby no separation in space between an active reactor and a sedimentation tank is needed, and which does not present the disadvantage of variable volumes, which results in a low use to total volume ratio, and which is therefore generally not cost effective for high flow rates, and intermittent influent feeding and effluent discharge.
This object is obtained by a wastewater treatment process or system according to the present invention, which process or system is a hybrid continuous flow, cyclic operating, substantially constant level, activated sludge process with a symmetric functional cycle.
The system combines all advantages of the conventional technology and the fill-and-draw, sequencing batch reactor (SBR) or cyclic operating technology without having of their disadvantages.
According to an embodiment of the invention, an activated sludge wastewater process for treating wastewater is provided comprising the steps of:
(a) in a first phase of operation: continuously receiving wastewater into a first compartment (A) of a series of fluidly linked compartments where it mixes with sludge received from a subsequent compartment of the series,
(b) passing the wastewater-sludge mixture along the series of compartments from the first compartment (A) of the series to subsequent compartments (B, C) of the series where internal substrates are further metabolised,
(c) passing the metabolised wastewater-sludge mixture to a last compartment (D) of the series where biomass is separated from effluent,
(d) discharging effluent from the current last compartment (D),
(e) preparing a compartment (C) of the series which is not the current last compartment to become a new last compartment for separation of biomass and effluent in a new phase of operation,
repeating steps (a) to (e) in the new operating phase, whereby all compartments (A, B, C, D) of the first phase become a series of fluidly linked compartments in the new phase with the new last compartment being used for separation of biomass and effluent. The new compartment for the separation compartment may be, for instance, the penultimate compartment or the first compartment in the preceding phase.
The biological reactor of the hybrid system to which this process applies comprises different hydraulically connected compartments for a single stage, which compartments are all identical, have the same equipment and follow the same functional cycle with reaction and sedimentation separated in time, while having constant level and continuous influent inflow and effluent outflow for both the overall system as for the hydraulically connected individual units. A separate sedimentation tank and sludge raking and recirculation devices are not needed. The reactor volume and reactor level in the identical tanks are constant.
The functional cycle arranges the reaction and sedimentation functions within the single embodiment separated in time. The reactor volume and reactor level in the identical tanks are substantially constant.
The symmetric functional cycle allows for symmetric (equal) influent distribution and therefore symmetric sludge and oxygen demand distribution. The functional cycle and the occurrence of identical functional units introduces a complete system redundancy; every tank can be separated from the other tanks by closing valves in interconnecting piping or channels, when maintenance is necessary.
The said functional cycle supports the introduction of substantial substrate gradients and promotes thus the formation of well settling flocks by the so called internal selector effect (high substrate/sludge ratio).
The said functional cycle supports optimal biological nitrogen and phosphorus removal by alternating anoxic (mixed), anaerobic (mixed) and aerobic (mixed, aerated) sequences in the activated compartment to promote denitrification, nitrification and the phosphorus metabolism in the activated sludge.
A biological reactor of the hybrid system according to the present invention comprises a plurality of units for a single stage process. The single stage process is generally applied for diluted to moderate concentrated industrial and municipal wastewaters.
All units or compartments of the biological reactor are hydraulically connected. There is no outer compartment nor middle compartment from point of view of functional cycle. The functional cycle can start or end at all compartments.
Combinations in series and parallel of said system are made depending on wastewater characteristics.
The said hybrid system is equipped with a core connecting infrastructure, containing interconnecting piping, automatic valves and monitoring and control equipment which together facilitate the functional operation of the said hybrid system.
The total hybrid system can be easily covered and/or built underground so that emissions and visual impact can be greatly reduced.
According to another part, the invention provides a continuous flow, constant level process for treating wastewater in a single reactor allowing alternate biological treatment and sedimentation, whereby sludge is recirculated to an infeed compartment by mechanical means such as e.g. a pump.
Although there has been constant improvement, change and evolution of wastewater treatment systems, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The description is given for the sake of example only, without limiting the scope of the invention.